Recently, as MIS transistors which form semiconductor integrated circuits have been scaled, gate oxide films have become rapidly thinner. Consequently, influences of depletion in the gate electrode (polycrystalline silicon film) near the interfaces between the gate electrode and gate oxide film caused when gate voltage is applied to a gate electrode to turn on a MIS transistor become more and more apparent. As a result, apparent thickness of the gate oxide film becomes thicker, which makes it difficult to have sufficient ON current, and operation speed of the transistor is significantly reduced.
Also, when the gate oxide film becomes thinner, since a quantum effect called direct tunneling occurs, which makes electrons pass through the gate oxide, the leakage current is increased. Further, in a p channel MIS transistor, boron in its gate electrode (polycrystalline silicon film) diffuses into the substrate through its gate oxide and impurity concentration of channel region is increased. Therefore, the threshold voltage fluctuates.
For its solution, the replacement of the gate insulator material from the silicon oxide to an insulating material with a higher dielectric constant (high dielectric constant material) and the replacement of the gate electrode material from the polycrystalline silicon (or polycide) to metal have been examined.
This is because, when the high dielectric constant film is used to form the gate insulator, the actual physical thickness can be increased by a factor of “dielectric constant of a high dielectric constant film/dielectric constant of a silicon oxide film” while maintaining the same capacitance of the equivalent silicon oxide thickness (EOT), and as a result, the leakage current can be reduced. As a high dielectric constant material, various metal oxides such as hafnium oxide and zirconium oxide have been examined. In addition, when a material not containing polycrystalline silicon is used to form the gate electrode, the reduction of the ON current due to the depletion and the boron leakage from the gate electrode to the substrate can be prevented.
In the case where gate electrodes are formed of metal materials, different metal materials are used for each gate electrode of an n channel MIS transistor and a p channel MIS transistor and work functions thereof are optimized so as to control the threshold voltage.
For example, Japanese Patent Application Laid-Open Publication No. 2000-252370 (Patent Document 1) discloses a CMOS circuit in which a gate electrode of an n channel MIS transistor is formed of zirconium or hafnium and a gate electrode of a p channel MIS transistor is formed of platinum silicide, iridium silicide, cobalt, nickel, rhodium, palladium, rhenium or gold.
Also, Japanese Patent Application Laid-Open Publication No. 2004-165555 (Patent Document 2) discloses a CMOS circuit in which a gate electrode of an n channel MIS transistor is formed of any one of titanium, aluminum, tantalum, molybdenum, hafnium and niobium and a gate electrode of a p channel MIS transistor is formed of any one of tantalum nitride, ruthenium oxide, iridium, platinum, tungsten nitride and molybdenum nitride.
Also, Japanese Patent Application Laid-Open Publication No. 2004-165346 (Patent Document 3) discloses a CMOS circuit in which a gate electrode of an n channel MIS transistor is formed of aluminum and a gate electrode of a p channel MIS transistor is formed of compound metal obtained by introducing a material having a work function higher than that of aluminum (for example, cobalt, nickel, ruthenium, iridium, platinum and others) into aluminum.
2005 Symposium on VLSI Technology Digest of Technical Papers p 230-p 231 (Non-patent document 1) discloses a phenomenon in which the work function of a gate electrode is reduced when a MISFET with a gate electrode formed of a metal film such as ruthenium on a gate insulator formed of hafnium oxide is annealed in an reducing atmosphere. It is considered that the reduction of work function is caused because Hf—O bonds in the hafnium oxide film are broken through the reduction treatment and the oxygen vacancy concentration is increased.